es-fluent

Derive macros and utilities for authoring strongly-typed messages with Project Fluent.

This framework gives you:

• Derives to turn enums/structs into Fluent message IDs and arguments.
• A cli to generate ftl files skeleton and other utilities.
• Language Enum Generation
• Integration via a embedded singleton manager or es-fluent-manager-bevy for bevy

Examples

• bevy (online demo)
• gpui

Used in

• koruma
• gpui-form
• gpui-table
• gpui-storybook

Installation

Add the crate with the derive feature to access the procedural macros:

[dependencies]
es-fluent = { version = "*", features = ["derive"] }
unic-langid = "*"

# If you want to register modules with the embedded singleton and localize at runtime:
es-fluent-manager-embedded = "*"

# For Bevy integration: replace `es-fluent-manager-embedded` with  `es-fluent-manager-bevy`
es-fluent-manager-bevy = "*"

Project configuration

Create an i18n.toml next to your Cargo.toml:

# Default fallback language (required)
fallback_language = "en-US"

# Path to FTL assets relative to the config file (required)
assets_dir = "assets/locales"

# Features to enable if the crate’s es-fluent derives are gated behind a feature (optional)
fluent_feature = ["my-feature"]

# Optional allowlist of namespace values for FTL file splitting
namespaces = ["ui", "errors", "messages"]

Incremental builds for locale assets

The manager macros discover locales at compile time. To ensure locale folder/file renames (for example fr to fr-FR) trigger rebuilds, enable the build feature of es-fluent in build dependencies and call the tracking helper from build.rs.

[build-dependencies]
es-fluent = { version = "*", features = ["build"] }

// build.rs
fn main() {
    es_fluent::build::track_i18n_assets();
}

Namespaces (optional)

You can route specific types into separate .ftl files by adding a namespace. All derive macros support the same namespace options:

EsFluent

use es_fluent::EsFluent;

#[derive(EsFluent)]
#[fluent(namespace = "ui")]
pub struct Button<'a>(&'a str);

#[derive(EsFluent)]
#[fluent(namespace = file)]
pub struct Dialog {
    pub title: String,
}

#[derive(EsFluent)]
#[fluent(namespace(file(relative)))]
pub enum Gender {
    Male,
    Female,
    Other(String),
    Helicopter { type: String },
}

EsFluentThis

use es_fluent::EsFluentThis;

#[derive(EsFluentThis)]
#[fluent_this(origin)]
#[fluent(namespace = "forms")]
pub enum Gender { Male, Female, Other }

#[derive(EsFluentThis)]
#[fluent_this(origin)]
#[fluent(namespace = file)]
pub enum Status { Active, Inactive }

#[derive(EsFluentThis)]
#[fluent_this(origin)]
#[fluent(namespace(file(relative)))]
pub struct UserProfile;

#[derive(EsFluentThis)]
#[fluent_this(origin)]
#[fluent(namespace = folder)]
pub enum FolderStatus { Active, Inactive }

#[derive(EsFluentThis)]
#[fluent_this(origin)]
#[fluent(namespace(folder(relative)))]
pub struct FolderUserProfile;

EsFluentVariants

use es_fluent::EsFluentVariants;

#[derive(EsFluentVariants)]
#[fluent_variants(keys = ["label", "description"])]
#[fluent(namespace = "forms")]
pub struct LoginForm {
    pub username: String,
    pub password: String,
}

#[derive(EsFluentVariants)]
#[fluent(namespace = file)]
pub enum StatusVariants { Active, Inactive }

Output Layout

• Default: assets_dir/{locale}/{crate}.ftl
• Namespaced: assets_dir/{locale}/{crate}/{namespace}.ftl

When namespaces are used, namespace files are treated as the canonical split for that locale. {crate}.ftl remains optional for backwards compatibility.

Namespace Values

• namespace = "name" - explicit namespace string
• namespace = file - uses the source file stem (e.g., src/ui/button.rs -> button)
• namespace(file(relative)) - uses the file path relative to the crate root, strips src/, and removes the extension (e.g., src/ui/button.rs -> ui/button)
• namespace = folder - uses the source file parent folder (e.g., src/ui/button.rs -> ui)
• namespace(folder(relative)) - uses the parent folder path relative to the crate root, strips src/ when nested, and keeps src for root module files (e.g., src/ui/button.rs -> ui)

If namespaces = [...] is set in i18n.toml, both the compiler (at compile-time) and the CLI will validate that string-based namespaces used by your code are in that allowlist.

Derives

#[derive(EsFluent)]

Turns an enum or struct into a localizable message.

• Enums: Each variant becomes a message ID (e.g., MyEnum::Variant -> my_enum-Variant).
• Structs: The struct itself becomes the message ID (e.g., MyStruct -> my_struct).
• Fields: Fields are automatically exposed as arguments to the Fluent message.

use es_fluent::{EsFluent};

#[derive(EsFluent)]
pub enum LoginError {
    InvalidPassword, // no params
    UserNotFound { username: String }, // exposed as $username in the ftl file
    Something(String, String, String), // exposed as $f1, $f2, $f3 in the ftl file
}

use es_fluent::ToFluentString;
let _ = LoginError::InvalidPassword.to_fluent_string();
let _ = LoginError::UserNotFound { username: "john".to_string() }.to_fluent_string();
let _ = LoginError::Something("a".to_string(), "b".to_string(), "c".to_string()).to_fluent_string();

#[derive(EsFluent)]
pub struct UserProfile<'a> {
    pub name: &'a str, // exposed as $name in the ftl file
    pub gender: &'a str, // exposed as $gender in the ftl file
}

use es_fluent::ToFluentString;
let profile = UserProfile { name: "John", gender: "male" };
let _ = profile.to_fluent_string();

#[derive(EsFluentChoice)]

Allows an enum to be used inside another message as a selector (e.g., for gender or status).

use es_fluent::{EsFluent, EsFluentChoice};

#[derive(EsFluent, EsFluentChoice)]
#[fluent_choice(serialize_all = "snake_case")]
pub enum Gender {
    Male,
    Female,
    Other,
}

#[derive(EsFluent)]
pub struct UserProfile<'a> {
    pub name: &'a str,
    #[fluent(choice)] // Matches $gender -> [male]...
    pub gender: &'a Gender,
}

use es_fluent::ToFluentString;
let profile = UserProfile { name: "John", gender: &Gender::Male };
let _ = profile.to_fluent_string();

#[derive(EsFluentVariants)]

Generates key-value pair enums for struct fields. This is perfect for generating UI labels, placeholders, or descriptions for a form object.

use es_fluent::EsFluentVariants;

#[derive(EsFluentVariants)]
#[fluent_variants(keys = ["label", "description"])]
pub struct LoginForm {
    pub username: String,
    pub password: String,
}

// Generates enums -> keys:
// LoginFormLabelVariants::{Variants} -> (login_form_label_variants-{variant})
// LoginFormDescriptionVariants::{Variants} -> (login_form_description_variants-{variant})

use es_fluent::ToFluentString;
let _ = LoginFormLabelVariants::Username.to_fluent_string();

#[derive(EsFluentThis)]

Generates a helper implementation of the ThisFtl trait and registers the type's name as a key. This is similar to EsFluentVariants (which registers fields), but for the parent type itself.

• #[fluent_this(origin)]: Generates an implementation where this_ftl() returns the base key for the type.

use es_fluent::EsFluentThis;

#[derive(EsFluentThis)]
#[fluent_this(origin)]
pub enum Gender {
    Male,
    Female,
    Other,
}

// Generates key:
// (gender_this)

use es_fluent::ThisFtl;
let _ = Gender::this_ftl();

• #[fluent_this(variants)]: Can be combined with EsFluentVariants derives to generate keys for variants.

#[derive(EsFluentVariants, EsFluentThis)]
#[fluent_this(origin, variants)]
#[fluent_variants(keys = ["label", "description"])]
pub struct LoginForm {
    pub username: String,
    pub password: String,
}

// Generates keys:
// (login_form_label_variants_this)
// (login_form_description_variants_this)

use es_fluent::ThisFtl;
let _ = LoginFormDescriptionVariants::this_ftl();